Boyfriend not wanting sex anymore?

My bf & I have been together for 3 years now. I know the puppy love stage is long gone, & that's completely fine. It's just that sometimes he'll go through "dry spells" where he doesn't want to do me for up to 2 weeks. SOMETIMES during the dry spells, he'll be in the mood more for porn than me. I know he watches it & what not & im fine with it. AND sometimes he just won't be in the mood for either. He always tells me he'll snap out of it, & he usually does within a week or two. When he does snap out of it we do it almost every day for a week or two, than it goes back to a "dry spell". This "dry spell" however has been especially hard for some reason. Maybe my sex drive is just extremely high this time, but it's frustrating to me when I'm with him all weekend & no sex, but the very 1st day I'm gone he'll do it to porn. I've talked to him about it & he tells me it's nothing to worry about, & that well most likely do it this weekend. He's confident nothing's wrong but I'm so not. Idk why I'm overthinking this all if this has happened to us before. It's just my biggest fear is porn being more important, but time after time he has told me it'll never be, nor will he ever do it to porn more than me. Am I just overthinking this all???
Is this normal in relationships??? Please help!